1. Field of the Invention
The present invention relates, generally, to magnetic flux-coupling type superconducting fault current limiters, more particularly, to an environment-friendly and semi-permanent magnetic flux-coupling type superconducting fault current limiter that has a primary coil winding and a secondary coil of the magnetic flux-coupling superconducting fault current limiter in series and superconducting element winding around the secondary coil in parallel so as to reduce a load on a element when a fault occurs, and to improve the handling and the time for recovering a fault current using linked flux as much as possible.
2. Description of the Related Art
As the superconducting fault current limiter uses a quench characteristic that an electric resistance does not exist at a normal state but becomes high immediately when a fault occurs, it can cut off a fault current rapidly without additional control devices and detecting devices, but also can be automatically recovered after the fault is removed. Various kinds of current limiters using a superconductor have been developed based on the above features.
The superconducting fault current limiter has been developed in consideration of an economic efficiency and reliability, so as to supply a high quality power to an electric power system. When a superconductor is used as a fault current limiting device, there is no need to replace it upon reintroduction of current into an electric power system and the increase of a required power via rapidly limiting and recovering the fault current. The resistive type current limiters that limit a fault current by switching operations out of these characteristics are classified into a shunt type, a transformer type and a magnetic flux-lock type. The magnetic flux-lock type current limiter, with a YBCO thin film as a fault current limiting element, can be easily diffused by modules is wound to a magnetic flux reactor has an effective structure owing to the increase of electric conducting currents and an active operation in accordance with the establishment conditions. Since a new superconducting current limiter that applies the principle of a magnetic flux type uses a ferromagnetic iron core as a magnetic flux medium, such as conventional magnetic flux-lock type superconducting fault current limiter, it is important to analyze characteristics of operations due to an inductance ratio in accordance with the primary winding and the secondary winding, define the conditions for designing the limiter.
These current limiters belong to a resistive type, which has a simple structure but can reduce a fault current rapidly when a fault occurs, and can be miniaturized easily. However, this limiter is very susceptible to damage of a current limiting element, because it is directly conductive. The shield inductive type is less susceptible to damage, because a fault current does not directly flow through it, but is not in an electric power system due to its large volume, for current manufacturing technology, which uses an iron core, and also due to the power loss of the iron core because it is manufactured in the shape of a tube or ring. Furthermore, a bridge type is designed to limit a fault current by the inductance of a superconducting coil using a diode for power, and does not create a quench of a superconducting element to maintain the limiting capability given repeated operations. It also has problems, such as the loss of a power element, the costs for manufacturing a superconducting coil and the increase of the volume of the limiter.
The magnetic flux-lock type as a quench type limiter has an electric resistance of 0, which is a feature of a superconductor, is similar to the resistive type limiter in that a fault current directly flows to a superconductor to generate a quench due to exceeding a critical current value, and then to limit a fault current. It also has features capable of overcoming the problems of the existing resistive type and the shield inductive type. The above-identified conventional magnetic flux-lock type superconducting fault current limiter is used as a part of main power lines between a power supplying terminal and a power receiving terminal and functions as a fault current limiting element when a fault is generated.
FIG. 1 is an equivalent circuit diagram of the conventional magnetic flux-lock type superconducting fault current limiter. The conventional magnetic flux-lock type superconducting fault current limiter, as shown in FIG. 1, has wires connected in parallel by winding the coil 1 (L3) and the coil 2 (L4) around the ferromagnetic iron core by N3 and N4 turns, respectively and to connect the superconducting element (RSC) with the coil 2 (L4) in series. At this time, the superconducting element (RSC) is inside a cooling bath containing liquid nitrogen, considering its critical temperature.
The operational characteristics of the magnetic flux-lock type superconducting fault current limiter are divided into an additive polarity winding, and a subtractive polarity winding, depending on the directions of winding the coil 1 (L3) and the coil 2 (L4), which are connected in parallel. If the voltages induced in the coil 1 (L3) and the coil 2 (L4) are represented as V3 and V4, respectively, the voltages of both coils are shown in Equations 1 and 2 as follows,
                              V          3                =                              N            3                    ⁢                                    ⅆ                              ϕ                3                                                    ⅆ              t                                                          Equation        ⁢                                  ⁢        1                                          V          4                =                              ±                          N              4                                ⁢                                    ⅆ                              ϕ                4                                                    ⅆ              t                                                          Equation        ⁢                                  ⁢        2            
The above magnetic flux-lock type superconducting current limiter has a voltage of ‘0’ at both terminals of the superconducting element (Rsc) at a normal state and therefore, the voltages at both terminals of the coil 1 (L3) and the coil 2 (L4) are represented as follows:
                                          (                                          N                3                            ∓                              N                4                                      )                    ·                                    ⅆ              ϕ                                      ⅆ              t                                      =        0                            Equation        ⁢                                  ⁢        3            
In Equation 3, if N3∓N4≠0, then
            ⅆ      ϕ              ⅆ      t        =  0.The voltages at both coils are not generated at a normal state to be maintained at 0V. However, if a fault occurs to cause a fault current in excess of the critical current to the superconducting element (RSC), the superconducting element (RSC) is quenched to dramatically increase the superconducting element resistance, which causes the linked flux of the coil 1 (L3) and the coil 2 (L4) to generate the voltages at both terminals of the coils and the fault current is rapidly limited.
However, the primary and secondary coils are connected in parallel with the superconducting element in a conventional magnetic flux-lock type superconducting current limiter. If a fault occurs, the voltage at both terminals of a superconducting element increases at the additive polarity winding, which gives a load to the superconducting element. Furthermore, if the superconducting element (RSC) has resistance of 0Ω at an additive polarity winding and the inductance ratio between the primary coil and the secondary coil is set to 1, a current where the superconducting current (ISC) flowing in the superconducting element is relatively much greater than the magnitude of the line current (IFCL) can circulate, and there is a problem that the turns ratio of the second coil to the primary coil cannot be set to be greater than 1.